Septic System D-Box FAQs

Questions & Answers on how to Install, Inspect, Test & Fix a Septic System Distribution Box

Septic D-Box Required or D-box Omitted FAQs

Do all septic tanks have a dbox?

Do all tanks have a dbox On 2019-04-07 by Anonymous

Reply by (mod) - no, not always

Typically yes but not necessarily always.

For example of septic tank that empties into a single septic effluent disposal line (unusal but possible ) or into a seepage pit doesn't need a D-box because only one exit line is leaving the tank.

As another example, though not a good design (as flow balancing adjustment isn't possible), someone might simply have used tees or wyes to lay out a septic soakaway bed with multiple lines but no single junction box or D-box.

My 30 year old septic has never given trouble, but has no D-box - Massachusetts says I need one. Really?

My Septic System was built in 1986 as specified in Engineering Title 5 Drawings. A D Box was specified between the Tank and A Leeching Tank.

Now I am trying to sell my Home of 30 years, and find that no D Box was provided.

The articles here indicate that the D Box is primarily a distribution System to serve a Leeching Tile Field of multiple lines. Why is a D Box required in a system that has never given me a single bit of trouble?

In Massachusets, I can not sell my home unless I tear up my New Driveway to put one in between Septic Tank and Leeching Tank. Please Help explain Law. (May 20, 2015) Anne MacAdam

Reply: yes

Anne,

"never given trouble" does not mean that sewage effluent has been properly disposed-of nor treated nor that it has not contaminated the environment.

More likely it means there was no visible problem at the ground surface. The features of a D_box and and why they are used in the article above.

For example the D-box allows both the distribution of effluent over several different soakbed trenches and it permits the balancing of that flow. It also gives a viewpoint to determine the condition of the fields, for example to see if there is a backup or an imbalance in flow.

The cost of repairing building defect such as a missing D-box is first, trivial as a percentage of the value of a home, less than one percent of the home's value in almost any imaginable circumstance so certainly should not get in the way of completing a sale,

and second, I'd worry that a system that was improperly installed in the first place might have other more costly but hidden problems.

Better to know that up front than to have a costly surprise or fraud litigation down the road.

If any of the articles in this series are not clear I'd be grateful for a specific question and I'd be glad to research or reply further.

When were D-boxes first used on septic systems?

Does anyone know when d-boxes started to be installed in homes in VA? For instance a house built in 1975, could it possibly be without a d-box? On 2017-02-11 by Homebuyer

Reply by (mod) -

Home

Distribution boxes have been installed on septic systems for more than 100 years, depending on the septic system design.

A septic design might not use a D-box in some installations even if the installer built a network of multiple leach lines.

However regulating the flow among those lines would be then impossible, and the construction of such a system is more-difficult as the proper slope has to be carefully maintained.

Septic D-Box Cover / Lid FAQs

Can I replace just the D-box lid?

I have a concrete d box with a plastic d box on the inside of it. The concrete lid was cracked and starting to crumble.

the rest of the concrete box looks ok and its lined with a plastic d box (Is this normal?) Should I replace the whole thing or just in stall the refabricated lid? 2016/04/12 Denny

Reply:

If the box is intact I'd replace the bad lid. Be sure it's water tight.

We can't get the lid off of the septic D-box

We have located our D box but the lip is sealed tight and we cant take the lid off. Any ideas to release the pressure - On 2017-05-11 by Dalton

Reply by (mod) -

Dalton,

Thank you for asking an important question about stuck d-box coverage.

It would be almost impossible for there to be a suction pressure that would be holding a d-box lid in place.

More likely a previous installer used a sealant or caulk to seal the lid against ground water leaking into the system. You may be able to pry the lid off by working carefully with a thin flat bar.

Septic D-box lid replacement cost

How much does it cost to replace the distribution box lid? On 2021-04-23 by Anonymous

Reply by (mod)

A septic D-box lid replacement cost depends on the type and size of the distribution box, but typically the part itself will cost between $15. and $250.

The cost to have the lid replaced depends on how much excavation is necessary to find the D-box and make it accessible.

If it's just a few minutes with a shovel, the cost ought to be a minimum service call fee from your septic company - which varies significantly depending on where you live.

But if someone has to bring a backhoe to your site and tear up the property to find (and hopefully not destroy) the septic drainfield piping and D-box, the cost will be at least several thousand dollars.

D-Box Flooding & Other Trouble Signs FAQs

Water in line between the septic tank and the dbox.

We have water in line between the septic tank and the dbox. What are potentials causes of this and solutions? Inspection showed tank and septic field as fine. Thanks. On 2021-07-12 y Drew oz

by danjoefriedman (mod)

@Drew oz,

Water in the line between the D-box and the septic tank, IF the septic tank effluent level is below the septic tank outlet pipe bottom, probably means that the D-box is blocked or damaged or the septic drainfield is flooded (failed) or there is surface runoff leaking into the D-box or its piping.

You'll want to be sure to read our advice at

SEPTIC D-BOX FLOODING - causes & cures for flooding & leaks at the septic system distribution box or Drop Box

let us know if after reading there you have questions or suggestions.

How do I fix leaks at a septic system D box

New septic put in 6 years ago. I have a pump chamber after my septic tanks (twin 750 gals), the chamber pump (100 or 150 gal ) kicks on when the level gets high enough. The grey water is pumped to my d- box about 25 feet. The d box has 2 outflow pipes leading to a leach field with baffles etc.

The d box seems level and the problem is no matter what we have used to keep the cover on the box ( a piece of blue stone 1 " thick as it sits in the middle of a brick patio ) the d box leaks. Again on the 4th of July with 12 -15 folks over ( part of my problem?) ,

I heard the pump chamber kick on, 15 seconds later could smell it and then about 3 to 5 ounces of water came out. The leaks came from the corners opposite the inflow.

I can't get it to stop leaking, every time the pump chamber kicks on. This is the 3 time in 6 years that I am faced with a dbox with a cover ( modified cover since its blue stone ) that leaks. If we cement the blue stone onto the d box how can this keep happening?

Can the water just "eat" the concrete seal over a year? pls let me know by email when there is a response to my question - - many thanks! (July 12, 2014) matt with the d box leak!

Reply:

Matt,

You could convert to a more-easily sealed D-box but I don't think that's the problem. I suspect that the D-box is too small AND that with the small size, the effluent (it's not graywater) is not being accepted into the drainfield rapidly enough.

The result is the pump is filling the D-box faster than its outflow rate, causing backup and odor complaints.

A much larger D-box, sufficient to receive and then drain by gravity into the drainfields the whole pump cycle volume would be one approach that may tempt you but I don't recommend it.

Rather, you need to look at the inflow capacity of the drainfield - it may be that the drainfield is undersized, or worse, that it is poorly designed / installed and is flooding, or possibly the line balancing openings in the D-box that balance flow into different drainfield lines are too small.

In sum, if you watch the D-box when the effluent pump cycles you'll probably see that around 125 gallons of effluent are surging into the D-box and overflowing it because the in-flow rate is faster than the outflow rate.

If the D-box overflows only at the very end of the pump cycle you could see if your pumping system rate can be adjusted to send effluent to the D-box more slowly.

Please see our article on Distribution box (drop box) install, inspect, troubleshoot & repair advice at D-BOX INSTALL REPAIR where I will include your question and our comments

When is it normal to see water in the septic D-box?

Does D box always have some water in it when not using system? On 2018-08-13 by Wayne

by (mod) - it's normal for some liquid in D-box

That would be normal if there is liquid in the bottom of the D-box - below the bottom of the outlet openings.

Watch out: If you see water above the D-box outlet openings then those drainfield lines are clogged and failing or are being flooded by rainwater or both.

Found High water in the D-box - has the D-box settled down?

Hello, I had a inspection done on my septic system for a pending sale, and the system was satisfactory but with a concern.

The concern was the water in the d-box is above the invert when running, the septic tank and field were satisfactory. what does this mean?

it does not appear to be clogged and they stated the d-box is operational and not leaking

? is it possible the d-box sunk and even though the effluent in running out it just is not out as fast as it should? On 2018-03-06 by David

Reply by (mod) -

David

When effluent in a D-box is above the bottom edge of the D-box outlet pipe(s) then the drainfield is indeed flooded and failing.

Saying that the "D-box is operational" sounds confusing.

For a D-box to "sink" placing liquid level over the drainfield outlet would not change evidence that there is standing effluent or water in the drainfield lines.

Is it ever OK to find solids in the D-box or septic distribution box

I have a gravity operated septic system under 6 years old. I had the tank pumped a few weeks ago because I'd noticed water making the driveway stay wet much longer on one side after heavy rain, nearest the septic field, and starting about halfway down it, about run 2 or 3 of 4.

The tank was full. But the problem is still present, so I uncovered the Distribution box to inspect it for the first time. It has flow levellers.

There is opaque dark brown liquid up to or close to the hole in each leveller. It looks scummy. There is no smell. Is there a problem? 20 January 2015 Gillian Andrew

Reply: NO

Gillian

In normal conditions a septic tank is always "full" - to the bottom of its outlet pipe. Pumping won't fix a thing though it may give a few days of relief.

If the drainfields are flooded the system needs replacement or repair or surface water or runoff diversion away from the soakaway beds.

If the D-box is full above its outlet levelers that's another clue that the fields are saturated.

If the D-box contains solids that's more disturbing news as it suggests that the septic tank outlet baffle is damaged or lost. Pushing solids into the soakbed significantly shortens its further life.

With no lid on the septic D-box water is overflowing out of it

Ok, i had a back up into my house. I had a company come out and pump the septic tank out (which was full of solids). They mentioned that they thought the distribution box might be clogged as well.

So a couple of days later we dug up the d-box. When we got to the lid on the dbox a hole accidentally was knocked into the lid and water started coming up from the dbox (the lid subsequently fell apart so a new had to be made). We pumped the dbox out but there were no solids in the dbox.

We snaked the line from the septic tank to the dbox as well as both distribution lines and found no blockages of any kind.

Then 3 days of solid rain hit the area and we basically pumped on a regular basis to keep the hole from filling up with water.

Now we have the lid finished but not installed back on the dbox. Is it normal in this situation, with the lid not on the dbox, for water to keep coming up from the dbox. Or is it a sign of another issue? (Jan 6, 2015) Jamie Steele

Reply:

Jamie

What you describe is an illustration of why pumping a septic tank will not fix a septic backup problem.

All it does is buy some time - until the tank is filled again, which could be hours or days. The backflow out of the D-box shows that the drainfield is saturated = flooded - not working.

Septic D-Box Replacement FAQs

Which D-box is better: concrete or plastic?

We are having to replace our leechfield and D Box. It's replacing a 50 year old system. The box is concrete. What is the difference between the concrete box and the plastic box. Which is better? On 2016-04-20 by Tina

Reply by (mod) -

Tina I actually don't think one product is better than the other, though the plastic or fiberglass D-box might have better built-in seals around connecting pipes while the concrete box might require application of a sealant. (no big deal)

Broken Concrete D-box replacement with a plastic Distribution Box?

I have a broken concrete distribution box and replaceing it with a plactic box can I leave the bottom of the broken box in place and put the new one on it for stability being it’s already solid and level On 2019-12-04 by Roger

Reply by (mod) -

Roger

I don't see any problem with that idea at all, as long as you can line up the incoming and outgoing pipes without messing up their relative elevations

Do we need a soil perc test performed when replacing a septic D-box?

Is a perc test needed to replace outlet pipe and distributor box on septic system? On 2019-03-06 by Oz

Reply by (mod) -

Oz that would be an odd requirement; you're simply repairing a damaged pipe and D-box, not moving nor rebuilding a drainfield.

But IF the inspector or installer suspects, by having seen sewage or flooding in the septic D-box, that the drainfield has failed, then indeed further investigation of the condition of the drainfield would be in order. I think a septic loading and dye test would be far more useful than a soil perc test in that case.

A soil perc test would be needed if your installer or engineer were planning to install a new / replacement septic drainfield.

Of course the final legal authority is always your local building department.

To understand this better I recommend reading

SEPTIC SOIL & PERC TESTS 

and comparing that with the

SEPTIC LOADING & DYE TEST PROCEDURE 

- Daniel Friedman

Is it common to replace the D-box when replacing a septic tank?

I got a quote for $12k to replace my septic tank. The contractor wants to charge more for the d box replacement because he couldn't anticipate it was bad.

I had another quote from a company I should have used that included the d box for the same price $12k. Is it common to replace the d box at the same time as the tank (55 year old system). On 2019-03-27 by Mike

by (mod) - not necessarily

Mike in my OPINION there is no reason that a septic contractor would automatically include a new D-box when replacing a septic tank, but it would make sense to have investigated the condition of the septic system to determine if the D-box is in good condition, since it's always less expensive to do the whole job at once than to have to return to a site.

Septic D-box Inspection FAQs

How often should the soakaway bed / septic field distribution box be checked?

At what age should the distribution box be inspected. (July 15, 2014) Anonymous

Reply:

Good question, Anon, for which I've not found an authoritative answer for these reasons:

OPINION: The need for & frequency of inspection of the D-box would vary by septic system design, local soil properties, level of usage.

For example in a system design that uses manually-set alternating soakaway bed field trenches the frequency would be at least that of the planned swap between fields - perhaps every 5 years.

Most people ignore the D-box and never look there until there is a sign of trouble. Then the box is located, opened and inspected for evidence of the septic field condition.

That's when defects such as tipping, leaks, or uneven outlet distribution are more likely to be discovered.

If I were to propose a D-box inspection frequency I'd suggest

1. Inspect any time there is other evidence of a septic system backup, failure, or effluent breakout

2. Inspect the D-box on a frequency specified by the septic system designer - if any was specified. For example alternating bed septic systems that use a pump or siphon system may specify annual inspection to check for equipment malfunction or for adjustment.

3. Inspect at the frequency of required drainfield area exchange / rest / modification

4. Inspect at 5-10 year frequency in other conditions

At our septic inspection the D-box was full. What does that mean?

Recently had house inspection and the D Box was found to be full. Please give me some reasons why the D Box did not distribute the water. The system is 30 years old. On 2019-04-18 by Pat

Reply by (mod) - flooding or solids in the D-box mean septic system failure

Pat

Watch out: Because the d-box or distribution box is basically a junction box connecting a single septic effluent input line into one or more outlet drain field lines if the box is flooded or even if it simply has liquid above the bottom of the outlet pipes than the drain field is either blocked or has failed.

Explain the 4-way piping connector (D-Box) used for septic drainfield soakbed layouts

I noticed that after my septic tank I come across a four way junction box of orangeburg piping.

It looks to me that the d box is layed out after this fourway. What would be the most logical explanation for this layout? (Mar 23, 2014) Rocky

Reply: the junction box is used to apportion septic effluent across multiple drainfield lines

Rocky,

Indeed in a typical septic tank and drainfield or soakaway bed installation, a single line exits the septic tank and connects to a distribution box or D-box that in turn feeds two or more outlets of piping that are routed into drainage trenches, galleys, or whatever.

But I cannot guess at how your piping is laid out. If that's what you're asking you'd start by guessing by taking a look at the size and shape of the available drainfield area.

In a companion article

SEPTIC DRAINFIELD SHAPE

we show several typical drainfield layouts.

Watch out: if your septic system piping uses Orangeburg pipe it's likely that it's past its reliable life and ready for replacement.

See details at ORANGEBURG PIPE 

Septic pumping company describes evidence of D-box failure and repairs needed

I recently had my septic tank pumped. When the D-box was checked, I watched the pumper as he briefly stuck his hand into the box. He told me I had a broken pipe and that it was passing rocks. He also said it was tipped.

He scheduled me for a drain field evaluation for $349.00. At first, I was stressed with worry about the outcome of all of this.

However, as I thought more about it, I'm now wondering if I'm being hustled. The pumper told me that there were sanitary products and baby wipes that had been flushed into the tank requiring him to do extra back washings. My adult daughter lives with me, but has never put these items down the toilet. On 2019-08-2 by Mary

Reply by (mod) -

While I agree that sanitary wipes or diapers should not be flushed down the drain and if they're going to septic tank they need to be removed, that's normally a part of any basic septic tank pumping and cleaning service. I can't imagine what extra work is being referred to but of course it's possible that there is something.

You septic pumper may not be an English major or clear communicator but the person not to be able to show you specifically what they're talkin about

Septic D-Box Elevation / Slope FAQs

Any tips on re leveling my existing distribution box?

Any ideas on re leveling my existing distribution box. I only need to raise one corner. On 2018-09-14 by (mod) - leveling the D box On 2018-08-05 by ted

by (mod) - bottomless D-box

Typically you'd shim the box level and assure the pipe connections are not leaking.

A stone, a bit of concrete, works fine along with some backfill.

My septic D-box has collapsed - top and sides are gone - missing for years!

Hi, I just dug up the pipe going from my septic to the D-Box, trying to find a leak (water coming out of the ground).

I found the pipe is ok, but the D-Box has collapsed. Black mud everywhere the D-Box should be. The only thing I could identify is the concrete bottom. The top and sides are gone. The pipes are still in place. I just had the septic pumped out and there was no sign of any sludge leaving the tank.

The D-Box may have been destroyed for years. It is in a part of the yard, that is never used and seldom mowed.

My question is, if I replace the D-Box, how do I clean out the field pipes (of mud) and test to see if they are ok ?
I was planning on cutting the output pipes back and then adding the needed pipe and coupler to get to the D-Box.

The pipes are pvc white pipe (not sure what schedule they are yet, or size (I believe 4", not measured yet)).
I just found the D-Box and it's condition an hour ago. I am sick over what I found and hoping I don't have to have a new drainage filed installed.
Any advice you can give will be greatly appreciated.

Also, I can get a plastic D-Box from nearby suppliers, but I will have to drive to another town to get a concrete box.
I am leaning towards getting the concrete box. What are your thoughts ?

(p.s. There are a lot of tree roots around where the D-Box is located. I am thinking they are what destroyed the D-Box.)

Thank you for your time. John in Virginia On 2018-08-08 by John Va

by (mod) - How to replace a destroyed or badly damaged septic D-Box

First, with a bit of measurement to see the size of the original D-box, you can simply have a new one installed. It'll take a bit more digging to make room to connect each of the existing pipes: the inlet from the septic tank and the outlets into the drainfield.

Plastic or concrete are either perfectly fine in my OPINION.

Watch out: you'll need to remove trees too close to the septic D-box and septic fields.

See PLANTS & TREES OVER SEPTIC SYSTEMS - how far away must shrubs & trees be from septic components

Watch out: also, given how long your D-box may have been missing and root invaded, I'm a bit pessimistic about the condition of the whole septic drainfield.

You might want to have the drainfield inspected further or to scope the lines for more root invasion.

See CAMERAS, SEWER / SEPTIC INSPECTION

Why is my septic D-box higher than the septic tank? Sewage does not flow uphill

Hi, while digging a trench I accidentally broke the main septic line between my Septic Tank and D-Box. The pipe was schedule 20 4" septic and was oblong. The pipe between the break and the D-Box was full of sludge. Also found out lid on D-Box was broken so it was plugged solid with dirt. I suspect this happened about 3 months ago while I was rototilling.

Was able to clean everything out and fix the broken line, however discovered that the D-Box is actually uphill from septic tank. It also appeared that the pipe was possibly cracked even before I hit it. Their was a lot of black soil underneath the break.

My guess is it happened 10 years ago when the original irrigation system was put in.

I know the tank is not full, so my question is what caused all the sludge build up in the pipe? Was it cracked pipe, the uphill grade, or the plugged D-Box? (Aug 25, 2014) Charlie

Reply:

Charlie

Oh for pete's sake: who the heck put the D-box uphill from the septic tank? The proverbial expression is "S_it doesn't flow uphill"

That would leave sludge in the pipe.

How is septic drainfield piping slope in and out of the D-box measured?

When measuring the 1/4" slope from the tank to the D box do you measure from the top ,middle or bottom of outlet On 2018-07-06 by Tom

by (mod) - where to measure slope of a septic drainfield pipe

thanks for the interesting question. It doesn't matter where you measure as long as you make the measurement at the same point on both ends of your Piper trench.

That is you can measure the bottom of the pipe where the top of the pipe to slope needs to be the same.

Septic D-box Location / Placement / Distances FAQs

What's the Maximum Pipe Length between Septic Tank and D-box?

What is maximum length of line between septic tank and diverter box ?

by Mod - maximum distance between septic tank and the D-box

David

There's not a code-specified maximum distance between septic tank and the D-box, but there will be a technical or practical distance limitation since the main effluent line from tank to D_box and from D-box to absorption trenches has to slope (typically 1/8" to 1/4" per foot).

A very long run that pushes drainfield lines too deep will have you building a system that lacks soil oxygen and thus doesn't treat the effluent adequately (not enough bacterial action) so then contaminates the environment, groundwater, wells.

Septic D-box is under a shed; can I splice in and install a new one somewhere else

My d-box is located under a large shed making access impossible. Is it possible to splice in and install a new d-box? On 2019-03-19 by Craig

by (mod) - yes with a lot of excavation and re-routing of septic effluent lines

Craig:

Technically you could splice in a new D-box elsewhere BUT you are likely to have to do an enormous amount of digging and there could be problems maintaining the require slope on the drain or effluent lines.

That's because currently it's likely that the various leachfield lines that are brought to the existing D-box run off in different directions.

You'd have to find each of those, intercept it, rout a new line to a new D-box and connect that to the septic tank, all the while keeping the slope of 1/8 to 1/4" per foot on the drains for the system to work.

It might make sense to open the floor of the shed.

Is a D-box ever ahead of the septic tank?

My tank has a d box attached to the tank before the sewage enters. Is there a reason for that. (Jan 19, 2016) scott

Reply:

Scott

Possibly to accommodate future additional sewer line connections or to give a cleanout access.

What's the allowable distance from septic tank to D-box?

How far can a septic distribution box be from the septic tank? My tank has two lids On 2018-02-12 by David

Reply by (mod) -

David

As long as you can keep the required line pitch (slope) of 4% and meet required property setbacks etc. I don't think you'll find an explicit distance limit. The D-box can be quite a distance away.

How far can the drainfield be from the D-box?

Is there a maximum distance the leach field can be from the d-box? On 2018-03-01 by Amy

by (mod) - maximum distance the leach field can be from the d-box?

Amy,

Not by code, but in a practical sense since you need to have an eighth to a quarter inch of slope per foot on the drain lines or leach field lines

or lines from the d-box to other leach field components, you can see that if the distance is quite great your Fields may be too deep below the surface of the ground to be either effective or economical.

How many septic D-boxes are there and do they all need to be replaced?

I am told that I need to replace 2 distribution boxes; however, everywhere I look I am seeing that a septic tank only has one box.

When the specialist came out the first time he told me he couldn’t find the D-Box. So he said he needed a special camera.

He can out a second time and told me Dbox #1 is used to transport fluid to Dbox #2 and Dbox #2 has 3 outlets lines.

That is when he informed me that both boxes need to be changed ($4300). Would love to understand if I am being taken advantage of, or if this is makes sense. On 2020-10-25 by Ilana

Answer by (mod) -

Inspection by camera has, fortunately, allowed you past speculative arm waving and into the world of moee useful facts.

The number of distribution boxes needed by a septic system varies and can range from one too many depending on the slope and shape of the terrain. So what you were told is perfectly reasonable.

What I don't understand and what is missing from what you've been told or were able to relate is the question of the condition of those distribution box is, why they all need to be replaced, and what the actual costs are.


That also varies by site.

Usually the main cost in replacing a distribution box is the cost of paying the excavator. So if a lot of Excavating is needed that msy expllain the cost.

Cost of the boxes themselves is small.

Septic D-Box Inlet & Outlet Opening Heights FAQs

Septic D-box inlet is lower than the D-box outlets: is that OK?

We are hearing gurgling from our bathroom closest to the septic; we opened the D-box to inspect, there are no clogs but noticed the inlet from the septic tank is lower than the outlets to the field. This doesn't seem normal but is it a problem? On 2016-08-20 by Dee

by (mod) - No

AT THE DISTRIBUTION BOX:

The inlet opening should be higher than the outlet openings in the D-box.

Otherwise the D-box has to flood before wastewater flows to the soakbeds.

Because only effluent, not solids, should be in the line between tank and D-box, this problem is not as critical as a backwards-plumbed septic tank (I'll describe that too in a moment).

Typically we see adjustments in the D-box to permit balancing of effluent flow among various leachfield (soakbed) lines or even to rest some of those laterals by shutting them off from time to time.

As long as the effluent is flowing freely into the soakbeds you're probably OK.

But if the D-box has to flood to its rim before it pushes effluent out to the fields then the septic tank may be back-flooding and not working properly itself.

For other readers:

AT THE SEPTIC TANK:

The septic tank outlet is normally higher than the tank inlet. If a tank is installed "backwards" as we sometimes find, it will not work properly.

For effluent to rise high enough to exit the backwards-plumbed septic tank to the absorption fields or soakbeds through the higher tank's inlet opening, the incoming wastewater, entering through the lower "exit" opening, will have to rise above that opening.

That tends to leave solids in the waste piping between building and tank and can result in a sewage backup in the building.

Is it OK to use elbows to lower the D-box outlet to the drainfield?

I had to replace my d-box and rather than having the leech line enter the box at it's current level (about 9 inches below the inlet), it seemed like a good idea at the time to use 2 elbows to create a drop, making the line enter the d-box slightly higher (about 2 inches below the inlet).

See pic. Now I'm questioning if this was a good idea. On 2018-09-22 by Todd

Reply by (mod) - shouldn't be necessary but may be OK

You should not have to modify the height of the outlet connections.

To be clear, in normal septic distribution box design the inlet line will be a bit higher than all of the outlet lines, and depending on how the various drainfield or soakaway bed lines are being used, some outlet openings in the D-box may be lower than others in order to adjust just how much effluent is routed into each of the various lines.

A more sophisticated D-box feature is the use of adjustable flow balancing covers that allow fine tuning of the flow of effluent into each of the d-box outlets.

So you should not have had to add a pair of elbows to lower the output effluent lines still further out of the D-box.

But as you've already done so, that may be perfectly fine, with one warning:

Watch out: lowering the outlet from the D-box, if it means that efflulent will no longer have proper slope (1/8 to 1/4" per foot) downwards to the drainfield will put your septic drainfield out of order.